A chloralkali process is a process that produces chlorine or a related oxidizer and an alkaline salt such as sodium hydroxide (“NaOH,” also known as lye and caustic). Chlorine and NaOH are among the most produced chemicals in the world and are used in the manufacturing of a wide range of materials and products.
An exemplary chloralkali process is illustrated in FIG. 1. The figure illustrates a typical brine electrolysis process 100 known to those skilled in the art using an electrolyzer. The electrolyzer of the illustrated typical brine electrolysis process 100 is a membrane cell 101. The membrane cell 101 includes an anode compartment 102, which contains an anode 103 and a cathode compartment 104, which contains a cathode 105. The anode and cathode compartments 102, 104 are separated from each other by a membrane 106. By way of example, the membrane 106 separating the anode and cathode compartments may be an ion exchange membrane. The membrane 106 separating the anode and cathode compartments may be operable to allow sodium ions and water to pass therethrough while preventing unreacted sodium chloride (NaCl) from entering the cathode compartment 104. A direct current 107 may be passed through the anode 103 and cathode 105. A stream 111 of saturated brine may be fed into the anode compartment 102 where chlorine from the NaCl is liberated at the positively charged anode 103. A portion of the chlorine, in the form of a gas, may be collected 112 from the anode compartment 102. Positively charged sodium ions from the NaCl migrate through the membrane 106 separating the anode and cathode compartments into the cathode compartment 105.
In the cathode compartment 104, hydrogen gas evolves from water molecules at the negatively charged cathode 105. The hydrogen gas may be collected 108 from the cathode compartment 104. The evolution of hydrogen from water also produces hydroxyl ions that react with the sodium ions to form NaOH. A portion of the NaOH is withdrawn 110 from the cathode compartment 104. Water may be added 109 to, and the NaOH may be withdrawn 110 from, the cathode compartment 104 to maintain desirable levels of NaOH in the cathode compartment 104. Accordingly, the overall reaction for the described chloralkali process is:2NaCl+2H2O→Cl2+H2+2NaOH
A depleted brine (e.g., brine no longer saturated with NaCl) stream 113 may be removed from the anode compartment 102. The depleted brine may be processed through brine processing 114 that prepares a saturated brine stream 111 to be fed into the anode compartment. Accordingly, a brine loop 115 comprises brine processing 114 to produce a saturated brine stream 111, feeding the saturated brine stream 111 into the anode compartment 102, the anode compartment 102, and removing depleted brine from the anode compartment 102 via a depleted brine stream 113 which is then fed back into the brine processing 114.
FIG. 2 illustrates a typical prior art brine loop 115 used in brine electrolysis. Hydrochloric acid (HCl) is added 201 to the depleted brine stream 113 removed from the anode compartment 102 to adjust the pH levels (e.g., increase acidity) of the depleted brine stream 113. This reduces the solubility of chlorine gas within the stream. The depleted brine stream 113 may then be subjected to vacuum dechlorination 202 where chlorine gas is drawn 203 from the depleted brine stream 113. A vacuum dechlorinated depleted brine stream 204 may be fed from vacuum dechlorination 202 and into chemical dechlorination 206. NaOH may be added 205 to the vacuum dechlorinated depleted brine stream 204 to adjust the pH upward (e.g., to make the depleted brine stream neutral or slightly alkaline). The NaOH may also help to stop gaseous chlorine from evolving from the dechlorinated depleted brine stream 204. The chemical dechlorination 206 may be achieved in a variety of ways known to those skilled in the art (e.g., by adding reducing agents such as sodium bisulfite (NaHSO3) and/or sodium sulfite (Na2SO3)).
After chemical dechlorination 206, the dechlorinated stream may be fed into a saturation step 207 where NaCl 208 may be added to create a saturated brine stream and water 209 may be added to replenish the volume of the stream and adjust the concentration of NaCl. Typically the NaCl 208 may include varying amounts of impurities that must be removed in order to run the membrane cell 101 at a high current efficiency. Major impurities typically include calcium, magnesium and sulfates. To remove these major impurities, the saturated brine stream may be passed through a precipitation process 210. This is typically a reactor or reactors where sodium carbonate (Na2CO3) and NaOH are added 211 to precipitate calcium carbonate (CaCO3) and magnesium hydroxide (Mg(OH)2). Depending on the particular impurities present, other reactions may be promoted.
The outflow of the precipitation process 210 may contain suspended solids from the precipitation process 210 and therefore is typically passed through a separation process 213. The separation process 213 may include the use of one or more gravity settlers, and/or one or more media filters including pre-coat and non pre-coat filters. The separation process may, for example, remove 212 precipitated CaCO3 and Mg(OH)2. The saturated brine stream may next be exposed to an optional activated carbon bed 214 to further remove any residual oxidizing materials. The saturated brine stream exiting the activated carbon bed 214, or the brine stream exiting the separation process 213 if an activated carbon bed 214 is not present, may still contain unacceptable levels of impurities. To further remove these impurities (e.g., calcium, magnesium, iron), the saturated brine stream may next be passed through an ion exchange process 215 that may include passing the saturated brine stream through a column containing an ion exchange resin. After the ion exchange process 215, the saturated brine stream 111 may be fed into the anode compartment 102 to complete the brine loop 115.
Known variations exist with respect to the above-described exemplary processes. For example, by altering process chemistry and temperature, the membrane cell 101 can be used to produce chlorate. It is also known by those skilled in the art that various steps as shown in the brine loop 115 may be added, altered or removed based on, inter alia, the quality of materials used in the process or manufacturing considerations. For example, in a particular brine loop, the activated carbon bed 214 may not be present, particularly if the levels of oxidizing materials in the brine stream after separation 213 are below a certain level. Furthermore, chloralkali processing may be achieved using, for example, mercury cells or diaphragm cells in place of the described membrane cells.